In a tertiary catalyst system empolyed as one of measures for public nuisance of automobile waste gas, an air/fuel ratio is controlled within a very narrow range of the equivalence point and an oxygen content after combustion is detected as an oxygen partial pressure by means of an oxygen sensor and then the detected signal is fed back to a control system of a fuel supplying unit. Therefore, the oxygen sensor is required to respond quickly to the oxygen partial pressure in the waste gas emitted from a heat engine of automobiles or the like. Namely, the oxygen sensor is required to exhibit a quick response time at least in the same degree as that shown by transmission property between a heat engine and a fluid medium with an efficient contact of the waste gas with an oxide. In the oxygen sensor detecting a change in the oxygen partial pressure of the ambient atmosphere as a change in the electric resistance, a transition-metal oxide is conventionally used for the material. The change in the electric resistance of the transition-metal oxide is caused due to variation in the stoichiometry of the oxide crystal and this change in the stoichiometry is induced through a possible contact with an oxygen in the waste gas. The response characteristic of the oxygen sensor is nothing but the response characteristic of the change in the electric resistance and the speed of the response of the stoichiometry or that of the change in the electric resistance of the oxide crystal is directly influenced by the contact efficiency between the waste gas and the oxide crystal, or the diffusion rate of the waste gas into the crystal. In general, the sensor element consisting essentially of such transition-metal oxide is formed into a disc shape by mixing an oxide with an organic binder solution, forming the mixture into green sheets by a doctor blade and then holding electrodes between the sheets for firing. Since, however, the element alone is mechanically weak, it must be assembled into a ceramic or metal support for practical use. Further, there is another disadvantage that the speed of the response of the oxygen sensor through the change in resistance of the transition-metal oxide is inferior to the conventional sensor wherein a change in the electromotive force of the zirconium oxide is utilized.